Combustion systems

ABSTRACT

An air inlet for an internal combustion engine comprises a plurality of tubes, each tube being so dimensioned that airflow therethrough changes from laminar to turbulent at a predetermined Reynolds number. Such an inlet is particularly useful in the combustion chamber of a gas turbine engine, where it can be used to slow down air velocity in engine-out flight conditions, while offering little resistance to flow when the engine is operating. The reduction in velocity assists in the engine relight procedure.

Waited ties Bryce tent n 1 COUSTION SYSTEMS [22] Filed: Mar. 19, 1971[21] Appl. No.: 126,158

[30] Foreign Application Priority Data Mar. 20, 1970 Great Britain..13,576/70 [52] US. Cl. ..60/39.65, 60/3974 R [51] Int. Cl. ..F02c 7/26[58] Field of Search ..60/39.65, 39.72 R, 60/3974 R, 249, 39.77, 39.09R; 123/142;

[56] References Cited UNITED STATES PATENTS 4/1951 Miller 60/396511/1952 Mock ..60/39.65

Way ..60/39.65

2,775,094 12/ 1956 Buckland ..60/39.65

2,825,203 3/1958 Bertin ..60/249 3,058,306 10/1962 Schlumbohm.....60/39.77 3,516,253 6/1970 Allport ..60/39.77

FOREIGN PATENTS OR APPLICATIONS 393,022 10/1908 France 123/142 642,2578/1950 Great Britain... .....60/39.65

674,840 2/1930 France 123/142 1,006,128 1/1952 France ..60/39.651,932,881 8/1970 Germany ...60/39.65

Pn'mary Examiner-Douglas Hart AttorneyCameron, Kerkam, Sutton, Stowell &Stowell [57] ABSTRACT An air inlet for an internal combustion enginecomprises a plurality of tubes, each tube being so dimensioned thatairflow therethrough changes from laminar to turbulent at apredetermined Reynolds number. Such an inlet is particularly useful inthe combustion chamber of a gas turbine engine, where it can be used toslow down air velocity in engine-out flight conditions, while offeringlittle resistance to flow when the engine is operating. The reduction invelocity assists in the engine relight procedure.

6 Clairm, 5 Drawing figures PAIENTEBAPR 1 015175 TRANSITION REGION.

TURBULENT REGION ENGIN E CONDITION S LAMINAR REGION mmOJ wmDmwwmaREYNOLDS NUMBER l2 l3 I4 IO M0 '3 IQ i H 00 0 FIG FIG.

COMBUSTION SYSTEMS This invention relates to air inlets for internalcombustion engines. Air inlets according to the invention areparticularly suitable for use in combustion chambers for gas turbineengines, especially in such chambers as used in engines for aircraftpropulsion.

The design of a gas turbine engine involves many compromises. Ingeneral, such engines are designed to be at their most efficient at aparticular condition, which in an aircraft engine is normally thealtitude cruise condition, and relative inefliciency has to be acceptedat other conditions. Problems of engine operation have also to beconsidered, and these tend to be greatest at conditions other than thedesign condition.

When the speed of a gas turbine engine is reduced, the air pressure, andhence the air density, within the combustion chambers of the engine arealso reduced. This reduction in density is associated with a reducedfuel flow, and the reductions in density and fuel flow must be such thata satisfactory fuel/air ratio is maintained. As a result of the reducedfuel flow the flame within the chamber becomes less stable. It sohappens that, in general, the corresponding reduction in air velocitythrough the chamber is relatively insignificant, and the danger of theflame being blown out by the airflow is therefore increased. Thisproblem is greater in an aircraft engine operating at altitude, wherethe air density in the combustion chamber is reduced compared withground level operating conditions.

Whilst engine controls have been developed to a stage where inadvertentflame extinction in an aircraft engine in flight is rare, provisionstill has to be made for relighting an extinguished engine. Also, insome instances, particularly in military operations, it is operationallyadvantageous to temporarily shut down one or more engines of amulti-engined-aircraft, and again provision must be made for enginerelighting.

When an engine is stopped in flight air will continue to flow throughit, due to the forward lI--of the aircraft, at a speed of the same orderas the speed of the aircraft. The air pressure and density within theengine combustion chamber or chambers will be considerably lower thanduring normal engine operation at the same speed and height. It will beclear from the above explanation that these conditions will makerelighting of the engine difficult, due to the tendency of the airflowto blow out the flame before it can get established.

This invention provides a form of air inlet to an internal combustionengine which compensates for reduction in air pressure by reducing theairflow velocity through the inlet, and which does so without the use ofmoving parts.

The invention makes use of the fact that when a fluid flows through apipe, the pressure loss per unit length due to wall friction is afunction of the pipe Reynolds number (Vd/v), where V is the entry flowvelocity, d the hydraulic diameter of the pipe, and v the kinematicviscosity of the fluid. At low values of Reynolds number the pipe flowis laminar and the pressure loss is high, whilst at high values ofReynolds number the flow becomes turbulent and the pressure loss isreduced.

According to the invention, an air inlet for an internal combustionengine comprises a plurality of tubes, each tube being so dimensionedthat airflow therethrough changes from laminar to turbulent or viceversa over a predetermined range of Reynolds number.

Each air inlet may be constructed by stacking and holding together aplurality of pipes, but a preferred form of inlet comprises a mesh ofstacked corrugated metal sheets.

Various embodiments of the invention as used with a combustion chamberof a gas turbine engine will now be described, by way of example only,with reference to the drawings, of which:

FIG. 1 is a graph (not to scale) showing the variation of pressure lossin a pipe with Reynolds number;

FIG. 2 is a schematic cross-section of part of a combustion chamber; and

FIGS. 3 to 5 are end views of three types of air inlet construction.

FIG. 1 illustrates the variation of pressure loss through a tube withReynolds number, and indicates the regions of laminar, transitional andturbulent flow and their relationship to typical engine operatingconditions.

FIG. 2 is a diagrammatic cross-sectional view of a combustion chamber.The chamber is symmetrical about the line li-II, and only one half isshown. The chamber has an outer casing l and an inner casing, or flametube, 2. Fuel is supplied through a sprayer 3 into air entering throughan entry duct 4. Swirl is imparted to the air entering through the duct4 by blades such as that shown at 5. Air is also admitted to thecombustion chamber through ducts 6, the primary function of this flowbeing to cool the metal surfaces of the chamber, and through holes, suchas that shown at 7, in the flame tube 2. The holes 7 are commonlyreferred to as primary combustion holes, and are designed to admit anamount of air sufficient to provide the correct fuel/air ratio in thechamber, at the design condition. Stacks 8, each comprising a pluralityof tubes, are shown in each of the ducts 4 and 6, and adjacent theprimary combustion holes 7.

Three alternative constructions of stacks 8 are shown diagrammaticallyin FIGS. 3 to 5. FIG. 3 shows a stack constructed from a plurality ofcylindrical tubes 9 bound, or bonded together. As shown in FIG. 4alternate sheets 1e, 11 of plain and corrugated material are bondedtogether to provide a plurality of tubes such as those shown at 12. Theconstruction shown in FIG. 5 is similar to that shown in FIG. 4, but inthis case corrugated sheets 13 are bonded together with the peaks of onesheet 13 contiguous with the troughs of the ad-' jacent sheet 13, togive a plurality of tubes such as those shown at 14.

The tubes 9, 12 or 14 are so dimensioned that the airflow therethroughis turbulent when the engine is running at normal operating conditionsand becomes laminar below a predetermined condition. It should beunderstood that the pressure in the combustion chamber is influenced byall the air inlets thereto, and that the effect on this pressure whenthe airflow through one stack 8 changes from laminar to turbulent, orvice versa, is small. The result of a change in airflow through a stack8 from turbulent to laminar will therefore be a marked reduction inairflow velocity through the tubes of the stack 8.

Design and positioning of the stacks 8 and of the tube dimensions willdepend on the result required. When the invention is used to facilitateaircraft engine relight procedure, flow conditions will be critical onlyin certain sectors of the combustion chamber, and a stack 8 in the duct4 may be sufficient to give the required result.

The tubes forming the stack 8 in duct 4 may be so dimensioned that theairflow therethrough is laminar when the engine is stopped with theaircraft in cruising flight at altitude, but becomes turbulent as soonas the engine is restarted and is running at its flight idling speed.With laminar flow the velocity of the air passing into the combustionchamber through the stack 8 will be low. Once the flow becomesturbulent, the pipe loss is considerably reduced and the adverse effectson engine performance are minimized. The swirler blades in the duct 4may be dispensed with by making the tubes 9, 12 or 14 curved, or byconstructing the stack 8 out of a series of sections, the sections beingarranged at various angles one behind the other to simulate curvedtubes.

Stacks 8 of tubes 9, l2, 14 may be advantageously used for other reasonsthan for that described above. For example, they may be used to controlthe velocity of the cooling air over combustion chamber surfaces, or inthe air passages of vaporizer units for gas turbine engines, where theairflow velocity tends to be undesirably high at low fuel flow rates.

What I claim is:

1. In a gas turbine engine in an aircraft designed for flight at highaltitudes, at least one combustion chamber comprising an outer casing,an inner casing,

means for supplying fuel to within the inner casing, means for ignitingthe fuel, and means for admitting air to within the inner casing, aproportion of the air being admitted to the vicinity of an outlet of thefuel supply means through an inlet comprising a plurality of tubes, eachtube being so dimensioned that airflow therethrough is turbulent duringnormal engine operation, and is laminar when the aircraft is in acruising flight condition at altitude with the fuel un-ignited.

2. A combustion chamber as claimed in claim 1 wherein the inletcomprises a mesh of stacked corrugated sheets, spaces between adjacentsheets forming the tubes.

3. A combustion chamber as claimed in claim 1 wherein the inletcomprises alternate sheets of plain and corrugated material, stacked andheld together, spaces between adjacent sheets forming the tubes.

4. A combustion chamber as claimed in claim 1 wherein the inletcomprises a plurality of pipes stacked and held together.

5. A combustion chamber as claimed in claim 1 wherein the inlet tubesare curved.

6. A combustion chamber as claimed in claim 1 wherein the inlet isconstructed from a series of sections, the sections being arranged atvarious angles one behind the other.

1. In a gas turbine engine in an aircraft designed for flight at highaltitudes, at least one combustion chamber comprising an outer casing,an inner casing, means for supplying fuel to within the inner casing,means for igniting the fuel, and means for admitting air to within theinner casing, a proportion of the air being admitted to the vicinity ofan outlet of the fuel supply means through an inlet comprising aplurality of tubes, each tube being so dimensioned that airflowtherethrough is turbulent during normal engine operation, and is laminarwhen the aircraft is in a cruising flight condition at altitude with thefuel unignited.
 2. A combustion chamber as claimed in claim 1 whereinthe inlet comprises a mesh of stacked corrugated sheets, spaces betweenadjacent sheets forming the tubes.
 3. A combustion chamber as claimed inclaim 1 wherein the inlet comprises alternate sheets of plain andcorrugated material, stacked and held together, spaces between adjacentsheets forming the tubes.
 4. A combustion chamber as claimed in claim 1wherein the inlet comprises a plurality of pipes stacked and heldtogether.
 5. A combustion chamber as claimed in claim 1 wherein theinlet tubes are curved.
 6. A combustion chamber as claimed in claim 1wherein the inlet is constructed from a series of sections, the sectionsbeing arranged at various angles one behind the other.